1. Technical Field
The present invention relates to a lamb type wave high frequency device used for communication apparatuses.
2. Related Art
High frequency devices are well known as devices included in high frequency resonators or filters. Typical examples of the high frequency resonator include surface acoustic wave elements using Rayleigh wave or an SH wave, and lamb wave elements using a lamb wave, which is a bulk wave. Some bandwidth filters also use the same waves as described above.
For example, a Rayleigh wave type surface acoustic wave element is known in which an interdigital transducer (IDT) electrode is formed in the Z′ axis direction, in which a surface acoustic wave is propagated, on the surface of a quartz crystal substrate called as an ST cut. The element is, for example, cited in a first example of related art.
A surface acoustic wave element using an STW cut quartz crystal substrate is also known, i.e. an SH wave type surface acoustic wave element in which a transverse wave propagated as a surface acoustic wave in the direction shifted by 90 degrees from the Rayleigh wave on the ST cut quartz crystal substrate. The element is, for example, disclosed in a third example of related art.
In addition to the surface acoustic wave, a lamb wave element is known that uses a bulk wave (a volume wave) propagating in a piezoelectric substrate by repeating reflections on the upper and lower surfaces thereof. The lamb wave element is known as the device particularly suitable for being used in high frequency since its phase velocity is faster than that of the surface acoustic wave. The element is, for example, sited in a second and fourth examples of related art.
In the lamb wave type element described above, it is known that a lamb wave can be efficiently excited using an AT cut quartz crystal substrate as a piezoelectric substrate, provided that the thickness H of the quartz crystal substrate and a wave length λ of a lamb wave are set in the range of 0<2H/λ≦10.
Analysis on frequency temperature characteristics of surface acoustic wave using finite element method, Technical Report of IEICE, US99-20 (1999-06), pp 37-42, Shigeo Kanna, is the first example of related art. Substrate for lamb wave type surface acoustic wave element, 33rd EM Symposium 2004, pp. 93-96, Yasuhiko Nakagawa, Masayuki Momose, and Shouji Kakio, is the second example of related art. JP-A-10-233645 is the third example of related art. JP-A-2003-258596 is the fourth example of related art.
The Rayleigh wave type surface acoustic wave element cited in the first example of related art hardly copes with a high frequency range since its theoretical phase velocity is about 3100 m/s even though it shows excellent frequency temperature characteristics as the surface acoustic wave element.
The SH wave type surface acoustic wave device cited in the first example of related art employs tantalum or tungsten having a density larger than that of aluminum as an electrode material to improve frequency temperature characteristics. However, it causes a disadvantage in that loss in electric resistance increases and further the phase velocity decreases.
The lamb wave element cited in the second example of related art can achieve high phase velocity (high frequency). However, the element has a disadvantage in that a desired resonance characteristic is not achieved due to so-called a spurious, in which another oscillation mode arises during the excitation of a selected oscillation mode since the exciting strength of an oscillation mode to be selectively used is low.
Here, disadvantages of known lamb wave elements will be described as a lamb wave type high frequency resonator as an example so as to further make clear advantages of the invention.
FIGS. 55A and 55B show a structure of known lamb wave type high frequency resonators. FIG. 55A is a perspective view illustrating a schematic structure. FIG. 55B is a cross-sectional view taking along the line Q-Q of FIG. 55A. In FIGS. 55A and 55B, a lamb wave type high frequency resonator 100 is composed of an interdigital transducer (IDT) electrode 130, and reflectors 140 and 150 that are provided at both sides of the IDT electrode 130, and a piezoelectric substrate 120 made of quartz crystal. The IDT electrode 130 and the reflectors 140 and 150 are fabricated on the surface of the piezoelectric substrate 120. The IDT electrode 130 includes a pair of interdigital finger electrodes 131 and 132. The reflectors 140 and 150 are disposed at both sides of the propagation direction of a lamb wave excited by the IDT electrode 130 and include electrode fingers (150a and 150b are shown as a representative). Hereinafter, the interdigital finger electrode 131 at one side is called as a first interdigital finger electrode 131, while the interdigital finger electrode 132 at the other side is called as a second interdigital finger electrode 132.
In the IDT electrode 130, the first interdigital finger electrode 131 and the second interdigital finger electrode 132, both of which are shaped in a comb-teeth like, are interdigitated. Here, the distance from the edge of an electrode finger 131a to an electrode finger 151b of the first interdigital finger electrode 131 is set as the wavelength λ of the lamb wave. Each width of the electrode fingers 131a and 131b is represented as Li.
The distance (may be called as a pitch) from the edge of the electrode finger 131a to the electrode finger 132a, which is interdigitated between the electrode fingers 131a and 131b, of the second interdigital finger electrode 132 is represented as Pi.
The distance (may be called as a pitch) from the edge of the electrode finger 150a to the electrode finger 150b in the reflector 150 is represented as Pr. Each width of the electrode fingers 150a and 150b is represented as Lr.
In the IDT electrode 130 and the reflectors 140 and 150, the pitches are set as to satisfy the relation of Pi=Pr, while the widths are set so as to satisfy the relation of Li=Lr.
Here, the following relation is satisfied: Hi=Hr, where Hi is each thickness of the electrode fingers 131a, 131b, and 132a of the IDT electrode 130, and Hr is each thickness of the electrode fingers 150a and 150b of the reflector 150.
The IDT electrode 130 and the reflector 140 are formed with the same electrode material such as aluminum (Al). Accordingly, the following relation is satisfied: pi=pr, where pi is the density of the electrode material of the IDT electrode 130 (may be called as the density of the IDT electrode), and pr is the density of the electrode material of the reflector 140 (may be called as the density of the reflector).
Next, impedance frequency characteristics, and conductance frequency characteristics of the lamb wave type high frequency resonator 100 structured as above will be described.
FIG. 56 is a graph illustrating an impedance frequency characteristic of the lamb wave type high frequency resonator 100 structured as above described (refer to FIG. 55). The horizontal axis shows frequency (presented by the value of f/fG, where fG is the maximum G frequency, and f is the frequency of the resonator), while the vertical axis shows the absolute value (unit is Ω) of impedance. As shown in FIG. 56, responses of a number of oscillation modes (hereinafter, may be simply referred to as a mode), i.e. A to F, of a lamb wave are confirmed in the lamb wave type high frequency resonator 100. This possibly results in other modes being exhibited as a spurious since they are not suppressed if the frequency of mode E is used for an oscillator, easily causing failures such as abnormal oscillations or frequency jumps.
Subsequently, reasons why an oscillation mode causing a spurious cannot be suppressed by known design will be described based on frequency characteristics of the radiation conductance and a reflection coefficient of the reflector.
Each of FIGS. 57 to 59 is a graph illustrating frequency characteristics of the radiation conductance and the reflection coefficient in respective modes B, C, and E in FIG. 56. The horizontal axis shows frequency (presented by the value of f/fG, where fG is the maximum G frequency, and f is the frequency of a resonator), while the vertical axis shows the radiation conductance (unit is S) and the absolute value of the reflection coefficient of the reflector.
Here, the following symbols represent as follows: G is the radiation conductance of the IDT electrode 130 only; Gres is the radiation conductance of whole lamb wave type high frequency resonator including the reflector; and Γ is the reflection coefficient of the reflector. In addition, the following terms are defined as follows: the maximum G frequency is the frequency when G shows the maximum; the maximum Gres frequency is the frequency when Gres shows the maximum; a reflection frequency band is the frequency range in which the absolute value of Γ is 0.5 and more; and a reflection center frequency is the frequency when the absolute value of Γ shows the maximum.
The larger G and Gres, the lamb wave is more intensively excited; the larger the absolute value of Γ, the lamb wave is more intensively reflected by the reflector. As shown in FIGS. 57 to 59, the maximum value of Gres is larger than the maximum value of G in modes B, C, and E, when the IDT electrode 130, and reflectors 140 and 150 are structured as above described. The reason why the maximum value of Gres in the modes B, C, and E become larger as described above is that the maximum G frequency is within the reflection frequency band. This means that each mode excited by the IDT electrode 130 is more extensively excited due to the efficient reflection by the reflectors 140 and 150.
As described above, according to the known technique, every mode shown in FIG. 56 is extensively excited by the reflectors 140 and 150, causing problems such as abnormal oscillations, and frequency jumps if the technique is employed to oscillator applications. In addition, if it is employed to filter applications, there arises a problem of ripples in frequency bands. Therefore, it is preferable that the maximum G frequency of mode excluding desired one is out of the reflection frequency band.